A power management circuit is a circuit which controls the power supply to at least one functional component of an electronic device. If a device comprises a plurality of functional components, a dedicated power management circuit may be associated to each of these functional components.
For example, a cellular terminal usually comprises a cellular modem as a first functional component, an application engine as a second functional component, etc. The term application engine refers to the “motor” of a terminal. It may comprise the application processor and related memory components, including the core software, hardware drivers, low level software and operating system, as well as related power management components and interfaces to peripherals like display, camera, keyboard, Bluetooth™ module, etc., even to the cellular modem. The application engine does not contain the mentioned peripherals, though. The idea behind the use of such an application engine is that it allows constructing many kinds of terminals using the same core components but varying for example the user interfaces, like display and keyboard. In general, it enables the construction of products that look quite different and have different features but that use the same technology inside.
Each of the functional components may be realized for instance on a separate chip, and each chip may comprise a power management circuit. Each power management circuit can be realized for instance in the form of an Integrated Circuit (IC) or an Application Specific Integrated Circuit (ASIC).
In case a plurality of power management circuits are present in a device, one of the circuits is normally defined to be a master circuit, while the other circuits are defined to be slave circuits. The master circuit controls the power-up and down of the entire system. This can be realized with an enable signal which is controlled by the master circuit and which can be detected by the slave circuits. Whenever the enable signal becomes active, the slave circuits cause the associated functional components to be powered up, and whenever the enable signal becomes inactive, the slave circuits cause the associated functional components to be powered down.
One criterion which is taken into account when controlling the powering state of an electronic device is the voltage level of a battery providing the device with a power supply. Normally, inserting a battery with a sufficiently high voltage into a device like a mobile phone causes the system to power up, even though this might not be visible to a user. Similarly, the system is powered up when the battery is empty, in case a charger is connected to the device and the battery voltage rises again to a sufficient level.
Typically, each power management circuit of an electronic device has its own battery voltage monitoring circuitry. The battery voltage monitoring circuitry monitors whether a sufficiently high battery voltage is available for regulators and other integrated components of the respective power management circuit.
Such battery voltage monitoring circuitry is frequently a comparator using hysteresis. When a higher power-up threshold is exceeded by the battery voltage, the associated functional components are powered up. When the battery voltage falls below a lower cut-off threshold, the associated functional components are shut down. With such a hysteresis, repeated power ups and shutdowns in an on/off oscillation can be avoided, even in case the battery voltage is close to a power-up threshold and varies for example due to a changing load.
For each power management circuit, the cut-off threshold has to be somewhat higher than the respectively highest output voltage of a linear regulator, in order to guarantee a proper voltage regulation. The cut-off threshold can be for instance about 100 to 200 mV higher than the highest regulator output voltage. By way of example, the nominal battery voltage could be 3.6 V and the highest regulated voltage could be 2.5 V. The power-up threshold could then be 3.1 V+/−100 mV, while the cut-off threshold could be 2.8 V+/−100 mV.
In order to avoid that some power management circuits shut down their associated functional components while other power management circuits are still powered up, all power management circuits could be designed to have the same cut-off threshold. But even if the cut-off thresholds are selected to be the same in all power management circuits, each power management circuit will still have a slightly different threshold due to an inherent inaccuracy. As a consequence, a controlled shut down is a problem. A corresponding problem arises when the system is starting up due to a battery voltage rising above the power-up threshold.
In a system with multiple power management circuits, the master power management circuit therefore usually also takes care of the battery voltage monitoring.
The battery value thresholds in all power management circuits have to be selected to this end such that the master power management circuit has the highest power-up threshold and the highest cut-off threshold. This guarantees that, when the master power management circuit powers up and activates the enable signal, all other power management circuits are already ready to operate. Similarly, this guarantees that the master power management circuit shuts down first and deactivates the enable signal, before any slave power management circuit starts to power off it associated functional components.
It is a disadvantage of such a system, though, that power management circuits having virtually the same battery voltage thresholds cannot be used as a master power management circuit and as a slave power management circuit within a single electronic device. Further, the cut-off threshold of the master power management circuit has to be unnecessarily high, in order to guarantee an appropriate voltage regulation in the slave devices. Thus, an efficient exploitation of the available battery capacity is prevented. Further, it is not possible to use any power management circuit as the master power management circuit. If the power management circuits are realized as ICs, the master power management circuit has to be fixed before the IC design freezes. Moreover, it is not possible, to use a slave power management circuit alone or as a master power management circuit, as the cut-off threshold might be too low for a voltage regulation.